1. Field of the Invention
The present invention generally relates to a method and apparatus for stacking sheets (e.g., metallized ceramic green sheets) used in producing laminated ceramic electronic components and, more particularly, to a method and apparatus for stacking large sheets (e.g., green sheets) having a size not less than 200.times.200 mm. For purposes of this application, "large" is defined as greater than 200.times.200 mm.
Generally, large green sheets can be processed more cost-effectively than standard size green sheets (e.g., sheets having a size of approximately 155 to 200.times.155 to 200 mm.) because of economies of scale and other factors. However, as green sheets become larger, manufacturing multilayered laminates becomes more difficult because of punching distortion, screening instability, handling loss and misalignment. The present invention provides a method and apparatus for reliably and precisely manufacturing large green sheet laminates without sheet distortion or misalignment.
2. Description of the Related Art
Typical multilayer ceramic electrical components, such as ceramic substrates and capacitors are formed from laminates (i.e., multiple sheets bonded together) of ceramic green sheets.
Green sheet laminates are typically produced by first forming a ceramic film in a continuous sheet and then drying the ceramic sheet. Thereafter, the sheet is punched into individual square or rectangular sheets. Various desired electrode patterns are formed and dried on the sheets, depending upon the electrical component to be produced, in a process called "screening". Then, the patterned sheets are stacked and bonded to form a laminate, and the laminate is cut into individual electrical components.
Specifically, the processing steps of transporting, stacking and bonding typically require the punched and patterned sheets to be transferred to a frame by a continuous strip carrier. Then, the sheets are loaded into a frame by a vacuum holder and are bonded together by heat and/or pressure. The sheets either are bonded one sheet at a time or many sheets may be bonded together simultaneously. Examples of such conventional processes are disclosed in European Patent Application Publication Number 0 530 052 A1 to Takaoka et al. and U.S. Pat. No. 5,174,842 to Hamuro et al.
The above conventional processes and apparatuses have several problems. For example, performing the required processing steps of transporting, cutting the green sheet and the carrier simultaneously, stacking and bonding the aligned patterned sheets correctly is crucial in producing defect-free electronic components. However, as mentioned above, such steps typically result in sheets which are misaligned with one another or sheets which are bonded insufficiently or improperly.
The misalignment results from inaccurate simultaneous shearing of a rigid carrier film and green sheet where there are unavoidably low tolerances between the punch and the frame holding the green sheet. The carrier film will also distort with the green sheet during the cutting thereof due to the nature of the high modulus carrier film that is used. This problem occurs, even though the purpose of the carrier film is to minimize the distortion and to increase stability. Also, the conventional full area individual thermal bonding process may distort the green sheet.
If sheets are misaligned or insufficiently bonded, the respective electrical connections between the sheets may be missed or only partially made, thereby resulting in short-circuiting problems and the like. Further, the capacitance of the electronic component may be inconsistent or outside the desired range. Also, internal electrodes may be severed during later cutting steps if they are improperly aligned. Therefore, conventional stacking methods and apparatus are not well-suited for large green sheet technology.